A disk drive system typically consists of one or more magnetic recording disks and control mechanisms for storing data within approximately circular tracks on a disk. A disk is composed of a substrate and one or more layers deposited on the substrate. In most systems, an aluminum substrate is used. However, alternative substrate materials such as glass have various performance benefits such that it may be desirable to use a glass substrate.
To produce a substrate from a blank sheet of a brittle material such as glass, the material may be scribed to generate a substrate having an inner diameter (ID) and an outer diameter (OD). One method of generating a disk substrate is to laser scribe a controlled shape, such as a circle, into one surface (the scribe side) of a glass sheet to generate the ID and OD contours of the disk substrate. A fine crack is propagated along the contours during the laser scribing. After scribing, the disk-shaped substrate may be removed from the excess bulk material of the sheet by breaking the material along the scribed contours.
Because of the radial deviation of the scribe, the non-scribed side of the substrate may have a poorly defined diameter. The ID may result in inwardly or outwardly protruding spurs, as illustrated FIG. 1A, and sloping side walls. Spurs on the surface of the disk substrate adjacent the ID may be generated when the glass is fractured to remove the disk from excess bulk material. Also, the scribe lines have a tendency to overlap resulting in imperfections to the disk substrate. The radial deviation of the laser scribe may also result in a substrate having a non-circular ID, as illustrated in FIG. 1B. The ID imperfections may be especially problematic because the ID is mounted onto the spindle of the disk drive.
Such disk substrate imperfections may result in a disk substrate that is not mass-balanced, as illustrated in FIG. 1C. The mass center may be at point A, while the rotational center (substantially the ID geometric center at which the disk rotates when mounted) may be at point B. A disk having this type of substrate may encounter balancing problems when rotated on the spindle of a disk drive system. In order to ensure proper rotation of a disk on a spindle, the mass center of a substrate should be located at the rotational center at which the substrate rotates. Thus, a mass balanced disk is one in which the mass center of the disk equates to the rotational center of the disk. A mass balanced disk is functionally important, because newer disk drive systems require higher rotational speeds. A high rotational speed of an unbalanced disk may lead to poor performance or disk failure. In addition, proper balancing is also necessary to achieve high track density by enabling the read/write head to accurately follow data tracks on a disk.
Another problem of disk substrate scribing results from the ID having a greater size than the drive spindle in order to fit properly over the disk drive spindle. A gap between the ID of the disk and the spindle diameter is present, as illustrated in FIG. 2, causing the disk to be located eccentrically on the drive hub. It should be noted that off angle illustration of the disk in relation to the spindle is not meant to imply a loss of planarity but, rather, to highlight the gap between the disk ID and the spindle hub. The gap results in an offset between the disk's rotational center and the spindle's rotation center. If the ID were scribed to fit exactly around the spindle, the disk would most likely be in balance. However, because of manufacturing limitations, such precision may not be practical.
Although disks that are not mass balanced during manufacturing could be corrected in post-production steps, current methods to correct disk imbalance may be costly and time consuming. Additionally, in multiple disk systems, mass balancing may be even more complex and costly compared to single disk systems.